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Aviation History
1946
1946 - 1007.PDF
MAY 23RD, 1946 FLIGHT Water Injection for Aircraft Engines PAPER BY M. R. ROWE G. T, LADD Results of Experiments by the Wright Aeronautical Corporation : Water and Water*Alcohol Additions WATER injection was the sub-ject of experiment in the auto-mobile industry as long as twenty years' ago, while in contrastthe virtually negative results of ex- periments with cars, carried out onlylast year, were recently revealed. For aircraft engines the requirements andoperating conditions are different, and a great deal of experimental anddevelopment work has been carried out, particularly in the United States,with a view to taking the best advan- tage of the cooling and anti-detonationeffects which can result from water or water-alcohol injection for petrol en-gines. Following is a summary of the prin-ciple, reasoning and conclusion con- tained in a paper on Water Injectionfor Aircraft Engines read before the Society of Automotive Engineers byMr. M. R. Rowe and G. T. Ladd of the Wright Aeronautical Corporation.This paper deals with water-alcohol mixtures as well as plain water, andthe term " antidetonant injection" is used synonymously with that of wateror water-alcohol injection. The authors state that: —" Basically, calculations have shown that the cylinder-head cooling ob-tained by using water or water-alcohol injection represents approximately 30to 40 per cent of the available heat of vaporization of the injected water.\ However, this heat of vaporization is ALCOHOL CONTENT OF A.D.I. FUEL 4O 6O 8OPERCENTAGE. VOLUME OF ALCOHOL IN VKTER 1OO 220 200 Jj ION-LIMITE D ) 0 ( z inn I \ 1 \ \ I\ j > J / J j so • i •O6 -O8 'K5 FUEL-AIR RATIO •12 Fig. 1. Freezing points of methyl andethyl alcohol solutions. Fig. 2. Typical effect of water injec-tion on detonation-limited i.m.e.p. for CFR engine. not considered to be the total resultanteffect of water injection. Test-result calculations on the compression ofgaseous fluids have demonstrated that the work exponential of the basicequation PVn = K will be reduced as water is injected into the work cycle,and that this reduction may amount to 10-12 per cent. This change indi-cates that the specific heat during the combustion cycle would have to betaken into account in any truly theoretical analysis of water injection. Peak Pressure Cooling ."In addition, calculations made atabsolute intake charge pressure con- ditions for constant speed, and in-creasing horsepower at constant cylin- der-head temperatures, show that theslope of specific brake water consump- tion does not change when going froma pressure condition of complete vaporization to that where the pres-sure is too high to permit vaporiza- tion. This is important and indicatesthat the main cylinder-head cooling effect is obtained during the high tem-perature and pressure peaks of the combustion cycle, since it does not ap-pear to matter whether the water enters the cycle as either vapour orliquid. Experience at the Wright Aero-nautical Corp. has taught that water and water-methanol mixtures are best suited for detonation inhibitors andinternal coolants. The addition of 50 per cent by volume of methanol towater is considered to be the optimum for water-methanol mixtures. Testswith pure alcohol, either methyl or ethyl, have indicated decreases in de-tonation-limited power outputs as compared to 100-octane fuel. Investi-gation of water vapour injection into induction air ahead of supercharginghas shown that this medium is an effective detonation inhibitor at leanfuel-air ratios only. '' Selection of the anti-detonant fluidfor addition to basic fuel flow is de- pendent upon three main factors: (1)The power output desired in excess of normal detonation-limited output; (2)the importance to the operators of engine critical altitude performance;(3) the ambient or atmospheric tem- peratures in which the aircraft is tooperate. '' Water alone offers the best rateof engine cooling, water-methanol mix- tures permit the greatest power out-put, and water-ethanol mixtures are inferior under both headings to eitherplain water or water-methanol. The freezing points of methyl and ethylalcohol solutions are shown in Fig. 1. Fortunately, the better anti-detonant ADI. WATER MIXTURES DRY WATER ETHYL -SO METHYL5O •O6 O8 -1O -12 FUEL-AIR RATIO Fig. 3. Effect of various anti-detonant fluids on detonation-limited i.m.e.p. for CFR engine.
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